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Abstract:

A method of cleaning fouled process equipment which includes a process
vessel (10) fouled by an organic foulant, includes spraying a hydrocarbon
stream at a pressure of at least 69 bar(g) at fouled surfaces inside the
process vessel (10) thereby to dislodge the organic foulant from the
fouled surfaces. The hydrocarbon stream is sprayed from at least one
nozzle (24) located inside the process vessel (10). The hydrocarbon
stream is at a temperature below the melting point of the organic foulant
or below the melting point of a major component of the organic foulant
when the organic foulant is a multi-component organic foulant. The
dislodged foulant is removed from the process vessel (10).

Claims:

1. A method of cleaning fouled process equipment which includes a process
vessel fouled by an organic foulant, the method including spraying a
hydrocarbon stream at a pressure of at least 69 bar(g) at fouled surfaces
inside the process vessel thereby to dislodge the organic foulant from
said fouled surfaces, the hydrocarbon stream being sprayed from at least
one nozzle located inside the process vessel and the hydrocarbon stream
being at a temperature below the melting point of the organic foulant or
below the melting point of a major component of the organic foulant when
the organic foulant is a multi-component organic foulant; and removing
the dislodged foulant from the process vessel.

2. The method as claimed in claim 1, in which the organic foulant
includes aluminium and in which the hydrocarbon stream includes, or is
defined by one or more hydrocarbons compatible with a catalyst of a
process employing the process vessel.

3. The method as claimed in claim 1, which includes displacing said at
least one nozzle from a retracted condition in which the nozzle is not
present inside, or not exposed to, a process volume defined by the
process vessel, to an operative or cleaning condition in which the nozzle
is positioned inside or exposed to the process volume defined by the
process vessel and capable of being aimed or directed at fouled surfaces,
including interior surfaces of the process vessel that are fouled.

4. The method as claimed in claim 1, in which the hydrocarbon stream
includes, or is defined by one or more hydrocarbons that are present in
the process vessel during normal operation of the process vessel.

5. The method as claimed in claim 1, in which the process vessel is an
oligomerisation or polymerization reactor.

6. The method as claimed in claim 5, in which the process vessel is an
ethylene oligomerisation reactor, and in which the hydrocarbon stream
includes C10+ hydrocarbons.

7. The method as claimed in claim 5, in which the process vessel is an
ethylene oligomerisation reactor, and in which the hydrocarbon stream
includes, or even consists predominantly of, linear hydrocarbons, as
opposed to C6 and heavier branched hydrocarbons.

8. The method as claimed in claim 1, in which the organic foulant is or
includes polyethylene and in which the hydrocarbon stream is at a
temperature of less than 105.degree. C.

9. The method as claimed in claim 1, in which said at least one nozzle is
displaced from its retracted condition in a compartment or parking space
closable or sealable from the process volume defined by the process
vessel, the method thus including opening a path between the compartment
or parking space and the process volume and displacing the nozzle along
the opened path from its retracted condition to its operative condition,
before spraying said hydrocarbon stream from the nozzle.

10. A process vessel adapted for cleaning, the process vessel defining a
process volume and including at least one displaceable nozzle operable to
be displaced between a retracted condition in which the nozzle is not
present inside or not exposed to the process volume and an operative or
cleaning condition in which the nozzle is positioned inside or exposed to
the process volume, the nozzle being connected to a high pressure
hydrocarbon stream supply line.

11. The process vessel as claimed in claim 10, which is an
oligomerisation or polymerization reactor.

12. The process vessel as claimed in claim 10, which is an ethylene
oligomerisation reactor.

13. The process vessel as claimed in claim 10, which includes at least
one stationary or fixed nozzle inside the process volume, the fixed
nozzle being connected or connectable to a high pressure hydrocarbon
stream supply line.

14. The process vessel as claimed in claim 10, which includes a
compartment or parking space for the displaceable nozzle, the compartment
or parking space being closable or sealable from the process volume
defined by the process vessel.

15. The process vessel as claimed in claim 10, in which the displaceable
nozzle is rotated in use by a hydrocarbon stream flowing through the
nozzle.

Description:

[0001] THIS INVENTION relates to the cleaning of process equipment,
including process vessels. In particular, the invention relates to a
method of cleaning fouled process equipment which includes a fouled
process vessel, and to a process vessel adapted for cleaning.

[0002] Many chemical processes are known which cause fouling of the
process equipment in use in these chemical processes. For example, during
oligomerisation or polymerization processes product mixtures consisting
of olefins, including alpha-olefins, C10+ olefins and polymer including
polyethylene may be produced. In the case of a tetramerisation or
trimerisation process, for example, ethylene is catalytically converted
into a product mixture consisting primarily of 1-octene and 1-hexene.
Typically, the product mixture will consist of at least 30% by mass
1-octene and 1-hexene respectively. However, during this reaction,
by-products consisting of light ends, C10+ olefins and polyethylene are
also formed. Polyethylene is therefore formed as a by-product and
although a large percentage of the polyethylene exits the reactor as a
slurry with the product mixture, the remaining polyethylene coats the
reactor surfaces. This fouling layer builds up over time and eventually
the reactor requires cleaning.

[0003] In the case of ethylene oligomerisation reactors, the cleaning of
the reactors has traditionally been effected by hot washing with a
solvent at elevated temperatures. Although highly effective, hot washing
has numerous undesirable consequences including thermal stress to the
vessel, long cleaning times, complicated agitator design and complicated
vessel internals.

[0004] Although hydroblasting of a process vessel, such as an
oligomerisation reactor, is a simple alternative cleaning method, the use
of water is very often undesirable as it may be a poison to any catalyst
used in the vessel. In addition, the time taken to open a process vessel
for cleaning is often also prohibitive.

[0005] According to one aspect of the invention, there is provided a
method of cleaning fouled process equipment which includes a process
vessel fouled by an organic foulant, the method including

[0006] spraying a hydrocarbon stream at a pressure of at least 69 bar(g)
at fouled surfaces inside the process vessel thereby to dislodge the
organic foulant from said fouled surfaces, the hydrocarbon stream being
sprayed from at least one nozzle located inside the process vessel and
the hydrocarbon stream being at a temperature below the melting point of
the organic foulant or below the melting point of a major component of
the organic foulant when the organic foulant is a multi-component organic
foulant; and

[0007] removing the dislodged foulant from the process vessel.

[0008] The organic foulant may include aluminium.

[0009] The hydrocarbon stream may include, or may be defined by one or
more hydrocarbons. The hydrocarbon or hydrocarbons defining the
hydrocarbon stream may be compatible with a catalyst of a process
employing the process vessel. In other words, if the process vessel is
used to process or contain a fluid which includes a catalyst, the
hydrocarbon or hydrocarbons of the hydrocarbon stream is/are preferably
not a catalyst poison and is/are preferably inert in relation to this
catalyst.

[0010] The primary cleaning mechanism of the method of the invention is
thus the forceful removal by a high pressure hydrocarbon jet or stream of
organic foulant from fouled surfaces. There is preferably very little,
more preferably no dissolution or chemical cleaning effect when the
method of the invention is applied. In other words, preferably the
organic foulant and the hydrocarbon(s) used for cleaning do not react to
any significant extent at the temperature of the hydrocarbon stream.

[0011] Typically, the dislodged foulant is removed from the process vessel
with the hydrocarbon used to clean the process vessel.

[0012] The method may include displacing said at least one nozzle from a
retracted condition in which the nozzle is not present inside, or not
exposed to, a process volume defined by the process vessel, to an
operative or cleaning condition in which the nozzle is positioned inside
or exposed to the process volume defined by the process vessel and
capable of being aimed or directed at fouled surfaces, including interior
surfaces of the process vessel that are fouled. Preferably, the nozzle is
displaced without having mechanically to open the process vessel, i.e.
without having to obtain access for personnel to the interior of the
process vessel from outside the process vessel.

[0013] The hydrocarbon or hydrocarbons defining the hydrocarbon stream may
be or may include a hydrocarbon present in the process vessel during
normal operation of the process vessel. As will be appreciated, by using
a hydrocarbon or hydrocarbons present in the process vessel during normal
operation of the process vessel, to define the hydrocarbon stream,
processing complexity is reduced and the risk that catalyst poison
ingress from an external source will take place is eliminated or at least
substantially reduced.

[0014] The process vessel may be an oligomerisation or polymerization
reactor. In one embodiment of the invention, the process vessel is an
ethylene oligomerisation reactor.

[0015] When the process vessel is an ethylene oligomerisation reactor, the
hydrocarbon stream may include hexene, octene and/or C10 and heavier
olefins.

[0016] Preferably, the hydrocarbon stream includes, or even consists
predominantly of, linear hydrocarbons, e.g. linear alpha-olefins such as
1-hexene or 1-octene, as opposed to C6 and heavier branched hydrocarbons
due to the higher diffusion rates and swelling power of molecules of the
linear hydrocarbons in downstream processing unit(s).

[0017] The hydrocarbon stream may be sprayed at a pressure of at least 345
bar(g). The pressure of the hydrocarbon stream may be in excess of 690
bar(g) or even in excess of 1380 bar(g).

[0018] The organic foulant may be, or may include a polymer, e.g.
polyethylene. When the organic foulant includes polyethylene, the
hydrocarbon stream is at a temperature of less than 105° C.,
preferably less than 80° C., e.g. between 20° C. and
70° C.

[0019] Said at least one nozzle may be displaced from its retracted
condition in a compartment or parking space closable or sealable from the
process volume defined by the process vessel and the method may thus
include opening a path between the compartment or parking space and the
process volume and displacing the nozzle along the opened path from its
retracted condition to its operative condition, before spraying said
hydrocarbon stream from the nozzle. The method may also include, after
the spraying of said hydrocarbon stream from the nozzle, displacing the
nozzle from its operative condition to its retracted condition along said
path, and closing or sealing the compartment or parking space from the
process volume. Typically, the compartment or parking space is external
of the process volume defined by the process vessel. For example, the
nozzle may be displaced from outside the process vessel through a valve
isolating the nozzle from the process volume, after the valve has been
opened to allow passage of the nozzle.

[0020] If desired, the method may include maintaining a pressure in the
compartment or parking space that is higher than the normal operating
pressure of the process vessel to ensure that material from the process
vessel does not ingress into the compartment or parking space.

[0021] Said at least one nozzle may be connected to a flexible hose,
displacement of the nozzle from its retracted condition to its operative
condition or vice versa respectively being by unwinding or winding of the
flexible hose. The nozzle may instead be attached to a telescopic lance
or deployed via a derrick.

[0022] The hydrocarbon stream may additionally be sprayed from at least
one fixed nozzle positioned inside the process vessel and aimed at a
surface particularly prone to fouling, e.g. an injector or an outlet
valve.

[0023] According to another aspect of the invention, there is provided a
process vessel adapted for cleaning, the process vessel defining a
process volume and including at least one displaceable nozzle operable to
be displaced between a retracted condition in which the nozzle is not
present inside or not exposed to the process volume and an operative or
cleaning condition in which the nozzle is positioned inside or exposed to
the process volume, the nozzle being connected to a high pressure
hydrocarbon stream supply line.

[0024] The process vessel may be an oligomerisation or polymerization
reactor, e.g. an ethylene oligomerisation reactor.

[0025] The process vessel may include at least one reactant inlet and at
least one product outlet and may in addition include an agitator. The
method of the invention is particularly useful when the process vessel
does not include an agitator or a pump around system which is typically
used to impart the necessary shear within the vessel during a hot wash.

[0026] The process vessel may include at least one stationary or fixed
nozzle inside the process volume. The fixed nozzle is typically aimed at
a surface inside the process vessel particularly prone to fouling. The
fixed nozzle is also connected or connectable to a high pressure
hydrocarbon stream supply line.

[0027] The high pressure hydrocarbon stream supply line may include a
flexible hose or telescopic lance as hereinbefore described. The nozzle
may be vertically displaceable inside the process volume defined by the
process vessel, to allow cleaning at any level in the vessel.

[0028] The process vessel may include a compartment or parking space as
hereinbefore described. The compartment or parking space may conveniently
be above the process volume defined by the process vessel, e.g. above a
lid or upper dome of the process vessel.

[0029] The displaceable nozzle is typically rotatable about at least one
axis, e.g.

[0030] a vertical axis or a horizontal axis. Preferably, the displaceable
nozzle is rotatable about at least two axes which are angularly spaced
relative to each other. Typically, the axes are 90° apart.

[0031] Preferably, the displaceable nozzle is rotated by a hydrocarbon
stream flowing through the nozzle. In other words, the displaceable
nozzle preferably does not include drive means, such as a mechanical or
electrical drive, to rotate the nozzle.

[0032] The invention will now be described, by way of non-limiting
example, with reference to the accompanying diagrammatic drawings in
which

[0033] FIG. 1 shows a partially vertically sectioned view of a process
vessel in accordance with the invention adapted for cleaning, prior to
cleaning of the vessel;

[0035]FIG. 3 shows more detail of cleaning equipment of the vessel of
FIG. 1; and

[0036] FIG. 4 shows a graph of removal depth as a function of stand-off
distance for water and diesel cleaning jets.

[0037] Referring to the drawings, reference numeral 10 generally indicates
a process vessel, in the form of an ethylene oligomerisation reactor, in
accordance with the invention. The vessel 10 includes an ethylene inlet
12, a product mixture outlet 14, an agitator 16, and a drain 15 with a
drain valve 17. A lid 18 is bolted to a flange 20 to close the vessel 10.

[0038] The vessel 10 further includes a cleaning head 22 comprising, in
the embodiment shown, two nozzles 24 aimed in opposite directions. The
cleaning head 22 is a conventional tank cleaning head which is rotatable
about a vertical axis, with the nozzles 24 being rotatable about a
horizontal axis. In use, the cleaning head 22, and the nozzles 24, are
driven to rotate by high pressure fluid (liquid hydrocarbon) flowing
through the cleaning head 22 and the nozzles 24.

[0039] The cleaning head 22 is suspended from a flexible hose 26.
Typically, the hose 26 is wound in the form of a hose reel 27 operated by
an electric motor (not shown).

[0040] The vessel 10 includes a compartment 28 attached to the lid 18 by a
valve 30 (typically a high pressure full port ball valve or a gate valve)
which provides access through the lid 18. The compartment 28 houses the
cleaning head when the vessel 10 is not being cleaned.

[0041] When desired to expose the cleaning head 22 to an interior space or
process volume 36 defined by the vessel 10, the valve 30 is opened to
allow the cleaning head 22 to enter the process volume 36.

[0042] In the embodiment illustrated in the drawings (cleaning of a CSTR
with an agitator), more than one compartment 28 and cleaning head 22 may
be used to prevent `shadowing` in the vessel 10. Shadowing is a
phenomenon known to those skilled in the art and is caused by the
agitator 16 or some other obstruction within the vessel 10 preventing
high pressure jets from striking the entire interior surface of the
vessel 10.

[0043] If necessary or desired, and if the compartment 28 is a closed
vessel or container, pressurising means may be provided to pressurize the
compartment 28, when the valve 30 is closed, with a fluid to ensure that
the compartment 28 is at a higher pressure than the process volume 36. As
will be appreciated, the compartment 28 may alternatively be pressurized
by means of the hose 26 and the nozzles 24.

[0044] The vessel 10 is used in conventional fashion to oligomerise
ethylene to produce primarily 1-octene and 1-hexene. During the
oligomerisation reaction, polyethylene is formed as a by-product and some
of the polyethylene coats interior surfaces of the vessel 10, as well as
the agitator 16. A fouling layer of polymer builds up over time and
eventually the vessel 10 and/or the agitator 16 requires cleaning.

[0045] In order to clean the vessel 10, the vessel 10 is drained (e.g.
using the drain 15 and the drain valve 17) and the valve 30 is opened
allowing the cleaning head 22 entry into the vessel 10 as shown in FIG.
2. The flexible hose 26 is thus unwound from the hose reel 27 so that the
cleaning head 22 drops from the compartment 28 through the valve 30 and
is located inside the process volume 36. A hydrocarbon stream of
1-octene, at a pressure of 550 bar(g), is sprayed through the flexible
hose 26 and the nozzles 24 of the cleaning head 22, in order to clean the
vessel 10. With the cleaning head 22 rotating about a vertical axis, and
the nozzles 24 rotating about a horizontal axis, a 360° cleaning
pattern over the entire interior surface of the vessel 10 is obtained. As
mentioned previously, `shadowing` can effectively be eliminated by using
more than one cleaning head 22 within the vessel 10. The exact number and
placement of the cleaning heads 22 are dictated by the vessel internals.
Optimum results can be obtained when no internals exist, e.g. in the case
of a bubble column where the use of only one compartment 28 and cleaning
head 22 is necessary in the centre of the vessel 10. By raising and
lowering the cleaning head 22, by means of the flexible hose 26, it is
possible to clean the entire interior surface of the vessel 10, including
the agitator 16. Dislodged polymer and the 1-octene used to dislodge the
polymer are removed from the vessel 10, through the drain 15.

[0046] Once the cleaning of the vessel 10 has been completed, the flexible
hose 26 is wound sufficiently around the hose reel 27 to retract the
cleaning head 22 into the compartment 28, and the valve 30 is closed.
During retraction of the cleaning head 22, the nozzles 24 are flushed
with the 1-octene stream to ensure that they are clean when they enter
the compartment 28.

[0047] Typically, a programmable logic controller (PLC) is used to control
the entire cleaning operation, and possibly also the oligomerisation
reactor or vessel 10. The PLC (not shown) can thus be programmed with
several types of fully automated cleaning programmes and will typically
be programmed to open and close valves, such as the drain valve 17, the
access valve 30 and a valve controlling the flow of the 1-octene, start
pumps, such as a pump for providing the 1-octene stream at high pressure,
raising and lowering the cleaning head 22 by means of the flexible hose
26, and the like. The PLC can also be programmed to ready the vessel 10
for start-up.

[0048] In order to test the cleaning method of the invention, an ethylene
tetramerisation reactor similar to the vessel 10 was used. The reactor
was fouled with polyethylene. The agitator was removed and a nozzle was
positioned in the centre of the reactor. The nozzle was configured to
rotate about the vertical axis and was moved slightly down the reactor
for each experimental run. The experimental runs were conducted with
water to show that high pressure is required to clean interior surfaces
of the reactor, but it is however to be borne in mind that water should
not be used for reactors employing a catalyst for which water is a
poison, such as an ethylene oligomerisation catalyst.

[0049] The following table sets out the experimental results for three
runs at different water pressures and flow rates and as can be seen, it
was possible to remove the polymer from fouled surfaces.

[0050] In order further to test and to compare the removal efficiencies of
water and a hydrocarbon, a further experiment was conducted. A 5 mm PVC-U
plate was used as a model compound to investigate the removal efficiency
of a high pressure jet of water versus a hydrocarbon jet at a similar
pressure. Diesel was used as a suitable model fluid for the hydrocarbon
stream. The PVC-U plate was positioned in a blast chamber fitted with a
robotic arm. A high pressure cleaning nozzle was attached to the end of
the robotic arm. The robotic arm was programmed to move across the PVC-U
plate at a speed of 50 mm/s whilst continuously increasing the stand-off
distance between the nozzle and the PVC-U plate from an initial distance
of 25 mm to a final distance of 500 mm. Two separate sweeps were carried
out along the length of the same PVC-U plate, the first used water as the
blasting fluid, whilst the second used diesel. A pressure of 700 bar(g)
was used with a 1 mm nozzle, yielding a volumetric flow rate of 0.27 l/s.
The removal depth (i.e. the depth of the cut into the PVC-U plate) was
then measured as a function of the stand-off distance between the nozzle
and the PVC0U plate for the two different fluids. Due to the lower
density of diesel, the mass flow rate of diesel was approximately 83% of
that of water. Since the cleaning force of the fluid was based on units
of mass and not volume, it was expected that the diesel would perform
slightly poorer than water.

[0051] FIG. 4 shows the measured removal depth as a function of the
stand-off distance of the nozzle from the PVC plate for water and for
diesel. It is clear from FIG. 4 that the water and diesel removed a
similar amount of material across the range of stand-off distances
tested. Water proved to be better up to distance of 300 mm, whilst,
surprisingly, diesel showed some improvement at distances greater than
300 mm, where the optimal efficiency was reached. Larger stand-off
distances are typical in real applications where the cleaning nozzle
needs to move freely within the process volume being cleaned. Thus, based
on the data of FIG. 4, diesel (or possibly other suitable hydrocarbons)
was established to be a suitable alternative to water as a high pressure
cleaning fluid.

[0052] Using the cleaning method of the invention, or a process vessel
adapted for cleaning in accordance with the invention, it is possible to
clean a fouled reaction vessel without opening the vessel to provide
access to personnel and without running the risk of allowing a catalyst
poison to enter the vessel. When the foulant is a polymer, the method of
the invention ensures that the polymer does not dissolve significantly in
the hydrocarbon stream used for cleaning as the temperature of the
hydrocarbon is below the melting point of the polymer. This prevents
dissolution of the polymer in the hydrocarbon thereby allowing for
minimal increase in the viscosity of effluent from the vessel being
cleaned. As will be appreciated, higher viscosities would lead to slower
drainage of the vessel during cleaning thereby impeding effective washing
of the lower portions of the vessel and increasing the time required for
cleaning of the vessel.

[0053] Importantly also, walls of the vessel are not exposed to thermal
stress as would be the case when a hot wash is used to clean the vessel.
The high pressure used in the cleaning method of the invention provides
the shear required to dislodge an organic foulant such as a polymer from
fouled surfaces. The combination of the hydrocarbon cold wash and the
high pressure employed ensures that the vessel is cleaned efficiently
without posing any dangers to the processes normally conducted within the
vessel.